The Claus process is used to produce sulfur of extremely good quality from a variety of gas streams containing hydrogen sulfide. Claus plants are widely used as adjuncts of gas-desulfurization installations to prevent discharge of large amounts of air polluting sulfur compounds to the atmosphere.
The first step of the Claus process is the combustion of a portion of the hydrogen sulfide in the feed stream in a flame zone at temperatures of 2000.degree.-3000.degree. F. to produce water, sulfur dioxide, and some sulfur. Sufficient hydrogen sulfide is converted to sulfur dioxide to yield a 2:1 molar ratio of hydrogen sulfide to sulfur dioxide by limiting the amount of oxygen entering the flame zone. This 2:1 molar ratio is the stoichiometrically correct ratio for the reaction: EQU 2H.sub.2 + SO.sub.2 .fwdarw. 2H.sub.2 O+ 3S (1)
to occur in the subsequent thermal and catalytic conversion zones of the Claus process.
Alternatively, all of the hydrogen sulfide in the feed stream is converted to sulfur dioxide in the flame zone, and then additional hydrogen sulfide is fed into the thermal conversion zone to yield a 2:1 molar ratio of hydrogen sulfide to sulfur dioxide.
The thermal conversion of sulfur dioxide and hydrogen sulfide occurs at temperatures above 1000.degree. F., preferably between 2000.degree. and 3000.degree. F. Since the combustion and thermal conversion of hydrogen sulfide is exothermic, producing approximately 290,000 BTU per pound of hydrogen sulfide oxidized to sulfur, the thermal conversion zone must be cooled to maintain the desired temperature. The cooling is accomplished with water, which is vaporized as low pressure steam e.g., 100-200 psig steam.
To remove the elemental sulfur formed in the flame and thermal conversion zones, the reaction products are cooled in a condenser below the dew point of sulfur. As much sulfur as possible is removed in the condenser to shift the thermodynamic equilibrium of subsequent reactions toward the production of additional sulfur and enable downstream catalytic conversion stages to operate at lower temperatures without material condensation of sulfur which poisons the catalyst.
After condensing sulfur from the gas exiting the thermal conversion zone, the gas is reheated to a temperature consonant with that required for feed to a catalytic conversion zone. In the catalytic conversion zone hydrogen sulfide from the feed stream reacts with sulfur dioxide produced in the flame zone in the presence of catalyst, usually alumina or bauxite, to produce water and sulfur. The reaction products again are cooled in a condenser, and the sulfur is removed. The steps of reheating, reacting, and condensing are repeated as often as three or more times in order to remove as much sulfur as is economically feasible or as may be required by air pollution standards. After leaving the last sulfur condenser, the exhaust gases are either incinerated to convert all sulfur compounds to sulfur dioxide and then vented, or treated to remove residual sulfur components as by U.S. Pat. No. 3,752,877 issued to me.
It has been proposed to use low pressure steam generated in the thermal conversion zone to preheat the feed to the catalytic conversion stages. While adequate to supply some of the heat requirements, this low pressure steam does not adequately satisfy all process needs. Other process needs include regeneration of the catalyst by removing sulfur, carbon, and hydrocarbon deposits. The sulfur is removed by stripping it from the catalyst with hot gas, and low pressure steam of 150 to 200 psig (358.degree.-382.degree. F.) is not adequate for preheating the hot gas for this purpose. Carbon and hydrocarbon deposits are removed by preheating the catalyst with a hot gas and then burning the deposits with a preheated combustible gas; to that end low pressure steam is inadequate to preheat the catalyst and combustible gas. It has been found that steam of at least about 600 psig is required to satisfy all process requirements.
Although these needs can be provided by generating high pressure steam in the thermal conversion zone, this approach results in a loss in overall efficiency due to insufficient conversion in the thermal conversion zone. This increases the load on the catalytic conversion zones, thereby requiring for some installations the addition of extra Claus stages which are expensive to install and operate.
When available from outside the process, some versions of the Claus process use higher pressure steam for the preheaters. Because higher pressure steam often is not available, the Claus units constructed under my direction use natural gas fired line burners to reheat the gas leaving the condenser. However, both of these approaches are wasteful of energy since they consume energy generated outside the Claus process without utilizing the energy available from the combustion of hydrogen sulfide. Furthermore, line burners are expensive, and can lead to upset conditions forming sulfur trioxide, which poisons the catalyst in the catalytic conversion zones.